The present invention relates to an electro-photographic color-image forming apparatus having a plurality of photosensitive bodies. More specifically, the invention relates to technology that detects positional displacements in each toner image formed on the photosensitive bodies carrying each color toner and transfers them with accurate positioning onto a recording material.
In the prior-art color-image forming apparatus employing electro-photography, an image has been typically formed through the following procedures. First, a photosensitive body is charged as an image-carrier by a charger. Second, the charged body is provided with light radiation according to image information to form an electrostatic latent image thereon. Third, the electrostatic latent image is developed into a visible toner image by a developing unit. Fourth, the visualized toner image is transferred onto recording materials such as a sheet of paper.
Some kinds of apparatuses employing tandem type color-image forming have been developed to respond to the need for color images. A tandem-type color-image forming apparatus has a plurality of image-forming stationsxe2x80x94a plurality of image-carriers. Each carrier is responsible for carrying cyan-, magenta-, yellow-, and preferably independent black-image. The four individual images are formed on the respective carriers in the series of image-forming process steps described above. All of the separately carried images are overlapped at a proper position of each carrier and transferred onto a sheet-type material to form a full-color image. Such a tandem type apparatus contributes to high-speed image forming due to the structural merit of having each image-forming section associated with each color. On the other hand, careful positioning (i.e., registration) of each image formed at different image-forming sections is indispensable for successful full-color image forming. Poor-registration of the four colors results in displacement in the whole picture or undesired gradations in color in transferring the image onto a sheet of paper or other materials.
FIGS. 34(a) through (e) illustrate typical displacements in transferred image. FIG. 34(a) shows displacement in the moving directionxe2x80x94shown by arrow A in the figurexe2x80x94of the transferred material (hereinafter referred to as displacement in secondary scanning). FIG. 34(b) shows displacement in the scanning directionxe2x80x94in a direction orthogonal to the direction indicated by arrow Axe2x80x94(hereinafter displacement in primary scanning). FIG. 34(c) shows displacement in a slanting direction (hereinafter called skew error). FIG. 34(d) shows scaling error, and FIG. 34(e) shows bend error. In a real world image-forming process, displacement is produced, with patterns FIGS. 34(a) through (e) complicatedly combined.
In the case of FIG. 34(a) above (displacement in secondary scanning), the displacement is mainly caused from inaccurate mounting of the image-forming stations or the scanning optical system, or inaccurate setting of the lenses and mirrors of the scanning optical system. This is also true in the case FIG. 34(b) (displacement in primary scanning).
The displacement described in FIG. 34(c) is caused from inaccurate angle-setting of the axis of rotation of a photosensitive drum in an image-forming station, or from inaccurate angle-setting of the scanning optical system. The displacement in FIG. 34(d) is mainly from inaccurate scanning length generated by the optical-path length from a scanning optical system to the corresponding photosensitive drum. The displacement in FIG. 34(e) is due mainly to inaccurate assembling of lenses in a scanning optical system.
In order to correct the displacement categorized in the five patterns, a suggestion has been made. The suggestion includes the steps of, (i) forming in advance a registration reference pattern (hereinafter referred to as a reference pattern); (ii) detecting positional displacement (displacement-detection) by a plurality of sensors; and (iii) performing the registration of each image (displacement-correction) according to the amount of displacement calculated from the result obtained in (ii).
Conventional reference-pattern detecting and displacement-correcting procedures will now be described.
FIG. 35 illustrates the structure of a prior-art reference pattern-detector (hereinafter pattern detector). FIG. 36 shows the layout of a reference-pattern formed on an inter-stage transfer belt and pattern detectors in the prior-art apparatus. FIGS. 37 and 38 illustrate the layout of a reference-pattern formed on an inter-stage transfer belt and pattern-detectors, and signals fed from the pattern detectors in the prior-art apparatus.
As shown in FIG. 35, pattern detector 14 contains an image sensor (hereinafter CCD) 51 and lens array 53 focusing light source 52 including a lamp, and reflected light onto CCD 51. This structured pattern detectors 14a and 14b are, as shown in FIG. 36, arranged so that a series of pixels in CCDs 51a and 51b are aligned in a line orthogonal to moving direction A of the inter-stage transfer belt (hereinafter simply referred to as belt) 12. Belt 12 has two CCDs, and each CCD is disposed close to a respective end of the belt in its widthwise direction with respect to the moving direction A of belt 12.
The reference pattern-detecting and displacement-correcting steps are performed based on a predetermined reference pattern formed of a line or figure pattern shown in FIG. 36. For example, the reference pattern can be formed of differently colored toner-images 54, 55, 56, and 57 at predetermined spaced intervals, and each toner image is disposed on a line orthogonal to the moving direction A of belt 12. Pattern detectors 14a and 14b detect positional displacement (i.e., registration displacement) based on an individual toner-image reference pattern.
In FIG. 37 (a), i) T1 represents the time required for each of the reference patterns 54, 5556, and 57 to reach CCD 51a in the pattern detector, ii) T represents the time predetermined as a design value, and iii) v represents the moving speed of belt 12. Then, displacement in secondary scanning shown in FIG. 34(a) is obtained by calculating displacement in individual color through the following equations:
xcex94T1=Txe2x88x92T1
xcex94Y1=xcex94T1xc2x7v
In FIG. 38(a), when passing CCD 51a in the pattern detector, the scanning-start position of each of reference patterns 54, 55, 56, and 57 on belt 12 has a difference in pixel position (represented by (X1). Displacement in primary scanning shown in FIG. 34(b) is obtained by calculating the displacement of the individual color based on the difference in pixel position.
Belt 12 has the same colored reference patterns 54, 55, 56, and 57 on both of its widthwise ends. A row of the reference pattern on one end passes CCD 51a, while the other row of the reference pattern in the other end passes CCD 51b. If the skew error (FIG. 34c) occurs, there should be a difference between the passing time of each reference pattern detected by CCDs 51a and 51b, as shown in FIG. 37(b). When the difference is represented by xcex94T2 and the moving speed of belt 12 is represented by v, the skew error is obtained from the following equation: xcex94Y2=xcex94T2xc2x7v.
If there is scaling error shown in FIG. 34(d), the scanning-start position and the scanning-end position of each of the reference patterns 54, 55, 56, and 57 on belt 12 have differences in pixel position when the two positions pass CCD 51a and CCD 51b, respectively. The differences are represented by xcex94X2, xcex94X1. The scaling error shown in FIG. 34(c) is obtained by calculation of the scaling error of each color based on xcex94X1, xcex94X2: xcex94X3=xcex94X2xe2x88x92xcex94X1.
After the four calculations described above, displacement correction is performed according to the amount of displacement as follows.
In the case of displacements in primary- and secondary-scanning shown in FIGS. 34(b) and 34(a), respectively, the amount of displacement is corrected by controlling the scanning-timing of each color.
In the case of the skew error and the scaling error shown in FIGS. 34(c) and 34(d), respectively, the amount of displacement is corrected by controlling the optical system in the exposure unit, using an actuator.
In the case of the bend error shown in FIG. 34(e), however, the amount of displacement cannot be determined with accuracy. Therefore, increasing accuracy in assembling lenses of the exposure unit copes with the error, instead of correction.
Through the procedures described above, the amount of displacement by color is detected then properly corrected according to the amount of displacement.
The cause of positional displacement, however, is not limited to the displacements categorized into five patterns above (hereinafter referred to as DC-component displacement). For example, irregularities in the thickness of the belt in the moving direction, an off-centered axis of a driving roller, an off-centered axis of a gear that drives the roller, an off-centered axis of a photosensitive drum, and an off-centered axis of a gear that drives the drum can also cause displacement. Therefore, even though the DC-component displacement has been corrected properly, a displacement caused by rotators including rollers, gears, and drums according to each rotation cycle, is not avoidable once they are driven to rotate.
For example, the displacement from an off-centered driving roller repeatedly occurs in the same page of a sheet material due to its incidence with a short cycle, while the displacement from variations in the thickness of the belt repeatedly occurs over pages of a sheet material due to its incidence with a long cycle.
Such a positional displacement caused by the rotation cycle of a rotator is referred to as AC-component displacement hereinafter.
It is therefore the object of the present invention to provide a color-image forming apparatus capable of minimizing a typical AC-component displacement, which is displacement in the toner image mainly caused by variations in the thickness of the inter-stage transfer belt.
In order to achieve this object, according to the color-image forming apparatus of the present invention, a plurality of image-forming stations contain a developing unit corresponding to the color, by which an electrostatic latent image formed on a photosensitive body is developed into a toner image. The exposure unit forms an electrostatic latent image on the bodies through light radiation.
An individually colored toner-image formed in a plurality of image-forming stations is transferred onto the inter-stage transfer belt one after another to obtain an overlapped image.
A plurality of the image-forming stations contain its own pattern detector. Each pattern detector detects displacement in toner image respectively formed in an image-forming station according to the registration reference pattern (hereinafter referred to as reference pattern) that is transferred one after another onto the belt.
The displacement corrector controls the image-forming timing provided by the exposure unit according to the result obtained from the pattern detector, that is, according to displacement information on the belt.
Such structured apparatus can minimize displacement in the toner image caused by irregularities in the thickness of the belt, correcting displacement on an image-forming basis.